The use of bacteria to treat cancer has been investigated for well over 150 years, and many genera of bacteria, including Clostridium, Bifidus, and Salmonella, have been shown to preferentially accumulate in tumor tissue and cause regression. The efficacy of such standard therapeutic strategies is limited because quiescent cancer cells can begin to proliferate and repopulate the tumor between courses of chemotherapy. The inability to completely kill all cancer cells following a single course of chemotherapy allows more time for individual cells to intravasate into blood vessels, increasing the chance of metastasis. Multi-drug resistance was observed over time, and bacterial therapies were reconsidered. Motile, nonpathogenic bacteria were believed to have the potential to overcome multi-drug resistance, with tumor penetration into quiescent cellular regions more effectively than otherwise possible using passively diffusing drug molecules.
Bacterial treatments expanded with the discovery of strains that specifically target tumor tissue. For instance, nonpathogenic Clostridium butyricum was used for the treatment of malignant brain tumors in mice. Intravenous injections caused noticeable regression due to accumulation in necrotic tumor regions. However, while complete regression of large tumors was observed, small tumors were unaffected. Moreover, the rate of recurrence was also unchanged, and animal death was imminent in all cases studied. The inability of Clostridia to alter rate of tumor recurrence is believed to directly relate to the fact that colonization of the tumor necrosis leaves a rim of viable cells at the tumor periphery. Even if lysis occurs, permanent eradication is not always guaranteed, as viable cells can continue to grow and ultimately repopulate the tumor.
An alternate approach became available through recombinant technologies. Clostridia were engineered into tumor-targeting vectors capable of delivering cytokines or prodrug-converting enzymes to poorly-perfused tumor regions. Anti-cancer therapeutics could then be produced locally within specific tumor regions, overcoming many of the diffusion limitation of systemically-administered chemotherapeutics. As an example, the cytosine diaminase gene of E. coli was cloned into a clostridial expression vector and transfected into C. biejerincki. In vitro assays showed that the bacteria were capable of producing high levels of the active E. coli derived enzyme. Used in combination, cytosine deaminase-expressing bacteria could be used to incite conversion of the non-toxic prodrug 5-fluorocytosine (5-FC) into the active chemotherapeutic agent 5-fluorouracil (5-FU) within specific tumor regions.
The use of Salmonella typhimurium to treat solid tumors began with the development of a nonpathogenic strain, VNP20009. Well-tolerated in mice and humans, this strain has been shown to preferentially accumulate (>2000-fold) in tumors over the liver, spleen, lung, heart and skin, retarding tumor growth between 38-79%, and prolonging survival of tumor-bearing mice. In initial clinical trials, S. typhimurium was found to be tolerated at high dose and able to effectively colonize human tumors.
Several strains of Salmonella have also been genetically modified to express the E. coli cytosine deaminase enzyme (TAPET-CD). When tumor-bearing mice were administered 5-FC in conjunction with VNP20009, accumulation of bacteria was 1000-fold higher than in normal tissue and tumor, and growth inhibitions of 88-96% were achieved. A pilot study on three refractory cancer patients was performed to test the accumulation and therapeutic affect of TAPET-CD in human patients. Proof of concept was demonstrated by the intratumoral conversion of 5-FC to 5-FU, demonstrating that Salmonella has inherent anti-tumor activity and the ability to deliver therapeutic enzymes and proteins to solid tumors in vivo. However, control over localization was not achieved and most Salmonella strains tend to colonize necrotic, rather than quiescent, regions.